Do You Need Synthetic Oil?

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Synthetic oils for marine engines can cost twice as much as conventional oils. But does your outboard or inboard really benefit from lab-spawned lubricants instead of those distilled from dead dinosaurs?

“We recommend a synthetic blend, a combination of synthetic and conventional oils,” says David Greenwood, product planning manager for Suzuki Marine (suzukimarine.com), which switched its company branded oil from conventional to a synthetic blend two years ago for its four-stroke outboards.

“Synthetics flow more smoothly on cold engine start-ups than conventional oils,” says Greenwood in pointing out one of the reasons for the change. “That really helps minimize engine wear.”

Full synthetics also perform great, but Greenwood maintains that synthetic blends work just as well and at a lower cost — about 25 percent less in most cases. Full synthetics really come into play when an engine is turning 8,000 rpm and up, says Greenwood, who explains that full synthetics are less likely to break down at high operating speeds and temperatures. Most marine engines don’t exceed 6,400 rpm, so using an expensive full synthetic amounts to a waste of money, Greenwood contends.

Synthetics also take longer to break down than conventional oils, but Greenwood says that this is fairly irrelevant since Suzuki’s oil-change schedule hasn’t changed since it was recommending conventional oils — every 100 hours.

Do the same rules apply to marine diesel engines? Frank Kelley, fuels and lubricants specialist for Mercury Marine (mercurymarine​.com), says full-synthetic oils help because they have fewer impurities that can oxidize and turn acidic when subjected to the higher temperatures inside a diesel engine with a turbocharger and circulating exhaust gases.

Mercury recommended conventional engine oil during its Cummins MerCruiser Diesel partnership (which was recently dissolved), but now recommends full-synthetic oil in its Volkswagen-based Mercury Diesel TDI power plants, Kelley says. “With the greater durability of full-synthetic oil, diesels can go longer between drain intervals,” he adds. Merc is targeting every 200 hours with the VW diesels.

Kelley agrees with Greenwood on the benefits of synthetic oil during cold starts. “The viscosity is such that, on a cold start, synthetic oil is more fluid and pumps more freely and gets to work more quickly,” he says.

However, Kelley doesn’t strongly favor synthetic blends. “By law, a company need only add 10 percent synthetic oil to conventional oil to call it a synthetic blend,” he explains. “It’s a bit of a marketing thing.” So if you go synthetic, go all the way to full synthetic, the Merc chemist asserts.

“Yet, what’s really important with engine oils — be it conventional or synthetic — is that boaters buy high-quality oils with good additive packages,” Kelley emphasizes. Also key is using the weights of oil specified by the engine builder, as well as adhering to the oil- and filter-change schedules, he adds.

“Finally, I recommend using brands from the engine builders,” Kelley says. “We want to make sure our engines always perform well, particularly in the warranty period, and we engineer our oils to help ensure that happens.”